1. Field of the Invention
This invention relates to a method for the formation of a dense calcium phosphate compound coating on the surface of a ceramic article.
Calcium phosphate compounds possess high bioadaptability and are useful as materials for artificial bones, artificial roots of teeth, etc. for implantation in the human body. When these compounds are used as said materials, they are in the form of sintered articles. It is widely held, however, that sintered articles of calcium phosphate compounds are deficient in strength, toughness, resistance against abrasion, etc. Bioadaptable ceramic substances also include alumina, zirconia, etc. These ceramic substances, however, are inferior to the calcium phosphate compounds in bioadaptability. High-strength materials excelling in bioadaptability can be produced by forming coatings of calcium phosphate compounds on ceramic substrates of alumina, zirconia, etc.
2. Description of the Prior Art
Sintered articles of calcium phosphate compounds exhibit high bioadaptability but low mechanical strength. Efforts have been heretofore made to impart added strength to these sintered article as by incorporating such high-strength materials as zirconia into calcium phosphate compounds or by orienting crystals of calcium phosphate compounds in one direction as in crystallized glass.
Such a bioceramic is described in U.S. Pat. Application Ser. No. 07/319,757, now U.S. Pat. No. 4,988,362, for example. Some of the inventors of this patent are also inventors of the present application. The allowed claim is as follows.
"A method for fusion of a bioceramic to the surface of a bioceramic inert to vital tissues, comprising:
(A) coating the surface of said bioceramic inert to vital tissues with a bioceramic composition consisting essentially of a composition produced by mixing alumina and silica in a gravimetric ratio in the range of 1:2 to 1:4 thereby preparing a mixed powder and incorporating into said mixed powder a calcium phosphate compound in an amount in the range of 10 to 60% by weight, based on the amount of said mixed powder, thereby forming a coating layer on the surface of said bioceramic;
(B) heating said bioceramic inert to vital tissues now provided with said coating layer thereby converting said coating layer into a solid solution containing aluminum phosphate formed by the reaction of calcium phosphate compound with alumina; and
(C) cooling the resultant of (B), thereby accelerating crystallization of the coating layer."
The invention defined by this claim still has the following problem.
The product of this fusion is inferior in bioadaptability and in bioaffinity to the material having calcium phosphate alone exposed on the surface because the coating layer of said product contains alumina, a substance deficient in bioadaptability, in a large amount at the surface thereof.
In this earlier invention, the calcium phosphate compounds rich in bioadaptability are represented by .beta.-Ca.sub.3 (PO.sub.4).sub.3, i.e. .beta.-tricalcium phosphate (hereinafter referred to as "TCP"), and Ca.sub.10 (OH).sub.2 (PH.sub.4).sub.6, i.e. calcium hydroxy apatite (hereinafter referred to as "HAp"). TCP and HAp, however, do not easily unite directly with ceramic substrates. It is known that plasma frame spraying, CVD, etc. can be used for coating ceramic surfaces and are capable of producing union of high strength. However, production of thick coatings by these methods requires much time and the use of special expensive devices.